The invention relates generally to electronic display apparatus and, more particularly, to apparatus incorporating multi-segment display devices.
Many types of electronic apparatus employ display devices to provide a readout of numeric data. One common type of display device utilizes seven short selectively energizable line segments arranged generally in a figure eight pattern. In order to display a specific character, selected segments are energized to form the desired character.
Each of the separate segments has its own electrode which, when energized in conjunction with a common electrode, causes that particular segment to light up, or in the case of liquid crystal displays, to block or transmit light. Additional apparatus or circuitry must operate in conjunction with the actual display device to interpret the desired numeric quantity and energize the particular electrodes associated with the line segments which must be activated in order to generate the desired character. The set of electrode energization patterns which will cause the desired characters to be formed on this type of display device is known as a seven-segment code. In order to provide the desired character to be displayed, the associated circuitry converts the input data signals into the seven-segment code. This is often accomplished by outputting data from a microcomputer to a separate seven-segment code converter and connecting this to the display. Since it is often desired to provide a multi-digit display, it is common to multiplex the digits such that each digit is rapidly activated in succession by a common seven-segment converter, sometimes ecapsulated as a single unitary multi-digit converter and display module.
Such conventional circuitry works well when only the ten numeric characters must be displayed. However, some applications require that a custom set of characters be displayed in addition to 0 through 9. For example, it is sometimes desired to indicate that the apparatus in question has detected an error condition, and to provide such indication by the display of a special character resembling an E or a backwards 3. In order to provide this and other non-numeric characters in a standard seven-segment display, a separate programmable read-only memory device (PROM) has been effectively utilized in certain applications. However, the use of such a device required extra circuitry. Some small bi-polar PROM's may consume as much power as a typical microcomputer. In addition, they require a tightly regulated 5-volt power supply and often must be operated near maximum output capabilities to provide sufficient current to drive the display device. These disadvantages can be avoided by using the technique of converting the display data to seven-segment code in the microcomputer and connecting the electrodes associated with each segment of the display device directly to the microcomputer. Such technique has been extensively utilized in calculator applications where size and cost are of prime importance. However, this technique requires 13 output lines from the microcomputer (one line for each of 7 segments plus 6 lines to select the desired digit) and in some applications sufficient microcomputer input/output lines were not available.
It would be desirable to provide apparatus incorporating a multi-digit seven-segment display device and a microcomputer for converting the display data to seven-segment code prior to output which would reduce the number of microcomputer input/output lines required.